diff --git a/lib/net/imap/command_data.rb b/lib/net/imap/command_data.rb
index 778517ac..33185d4c 100644
--- a/lib/net/imap/command_data.rb
+++ b/lib/net/imap/command_data.rb
@@ -27,6 +27,7 @@ def validate_data(data)
         end
       when Time, Date, DateTime
       when Symbol
+        Flag.validate(data)
       else
         data.validate
       end
@@ -47,7 +48,7 @@ def send_data(data, tag = nil)
       when Date
         send_date_data(data)
       when Symbol
-        send_symbol_data(data)
+        Flag[data].send_data(self, tag)
       else
         data.send_data(self, tag)
       end
@@ -138,11 +139,13 @@ def send_list_data(list, tag = nil)
     def send_date_data(date) put_string Net::IMAP.encode_date(date) end
     def send_time_data(time) put_string Net::IMAP.encode_time(time) end
 
-    def send_symbol_data(symbol)
-      put_string("\\" + symbol.to_s)
-    end
-
     CommandData = Data.define(:data) do # :nodoc:
+      def self.validate(...)
+        data = new(...)
+        data.validate
+        data
+      end
+
       def send_data(imap, tag)
         raise NoMethodError, "#{self.class} must implement #{__method__}"
       end
@@ -158,8 +161,26 @@ def send_data(imap, tag)
     end
 
     class Atom < CommandData # :nodoc:
+      def initialize(**)
+        super
+        validate
+      end
+
+      def validate
+        data.to_s.ascii_only? \
+          or raise DataFormatError, "#{self.class} must be ASCII only"
+        data.match?(ResponseParser::Patterns::ATOM_SPECIALS) \
+          and raise DataFormatError, "#{self.class} must not contain atom-specials"
+      end
+
       def send_data(imap, tag)
-        imap.__send__(:put_string, data)
+        imap.__send__(:put_string, data.to_s)
+      end
+    end
+
+    class Flag < Atom # :nodoc:
+      def send_data(imap, tag)
+        imap.__send__(:put_string, "\\#{data}")
       end
     end
 
diff --git a/test/net/imap/test_command_data.rb b/test/net/imap/test_command_data.rb
index ba1479bc..ac0047f9 100644
--- a/test/net/imap/test_command_data.rb
+++ b/test/net/imap/test_command_data.rb
@@ -6,6 +6,8 @@
 class CommandDataTest < Net::IMAP::TestCase
   DataFormatError = Net::IMAP::DataFormatError
 
+  Atom = Net::IMAP::Atom
+  Flag = Net::IMAP::Flag
   Literal = Net::IMAP::Literal
   Literal8 = Net::IMAP::Literal8
 
@@ -60,6 +62,66 @@ def send_data(*data, tag: TAG)
     @imap = FakeCommandWriter.new
   end
 
+  test "Atom" do
+    imap.send_data(Atom[:INBOX], Atom["INBOX"], Atom["etc"])
+    assert_equal [
+      Output.put_string("INBOX"),
+      Output.put_string("INBOX"),
+      Output.put_string("etc"),
+    ], imap.output
+
+    imap.clear
+    # atom may not contain atom-specials
+    [
+      "with_parens()",
+      "with_list_wildcards*",
+      "with_list_wildcards%",
+      "with_resp_special]",
+      "with\0null",
+      "with\x7fcontrol_char",
+      '"with_quoted_specials"',
+      "with_quoted_specials\\",
+      "with\rCR",
+      "with\nLF",
+    ].each do |symbol|
+      assert_raise_with_message(Net::IMAP::DataFormatError, /\batom\b/i) do
+        imap.send_data Atom[symbol]
+      end
+    end
+    assert_empty imap.output
+  end
+
+  test "Flag" do
+    imap.send_data(Flag[:Seen], Flag[:Flagged],
+                   Flag["Deleted"], Flag["Answered"])
+    assert_equal [
+      Output.put_string("\\Seen"),
+      Output.put_string("\\Flagged"),
+      Output.put_string("\\Deleted"),
+      Output.put_string("\\Answered"),
+    ], imap.output
+
+    imap.clear
+    # symbol may not contain atom-specials
+    [
+      :"with_parens()",
+      :"with_list_wildcards*",
+      :"with_list_wildcards%",
+      :"with_resp_special]",
+      :"with\0null",
+      :"with\x7fcontrol_char",
+      :'"with_quoted_specials"',
+      :"with_quoted_specials\\",
+      :"with\rCR",
+      :"with\nLF",
+    ].each do |symbol|
+      assert_raise_with_message(Net::IMAP::DataFormatError, /\bflag\b/i) do
+        imap.send_data Flag[symbol]
+      end
+    end
+    assert_empty imap.output
+  end
+
   test "Literal" do
     imap.send_data Literal["foo\r\nbar"]
     imap.send_data Literal["foo\r\nbar", false]
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index ebd926fb..d2aa9bb3 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -622,6 +622,34 @@ def test_send_invalid_number
     end
   end
 
+  def test_send_symbol_as_flag
+    with_fake_server do |server, imap|
+      server.on "TEST", &:done_ok
+
+      imap.__send__(:send_command, "TEST", :Seen, :Flagged)
+      assert_equal "\\Seen \\Flagged", server.commands.pop.args
+
+      # symbol may not contain atom-specials
+      [
+        :"with_parens()",
+        :"with_list_wildcards*",
+        :"with_list_wildcards%",
+        :"with_resp_special]",
+        :"with\0null",
+        :"with\x7fcontrol_char",
+        :'"with_quoted_specials"',
+        :"with_quoted_specials\\",
+        :"with\rCR",
+        :"with\nLF",
+      ].each do |symbol|
+        assert_raise_with_message(Net::IMAP::DataFormatError, /\bflag\b/i) do
+          imap.__send__(:send_command, "TEST", symbol)
+        end
+        assert_empty server.commands
+      end
+    end
+  end
+
   test("send PartialRange args") do
     with_fake_server do |server, imap|
       server.on "TEST", &:done_ok